Vertical motion impeller-type shot peening device and coil spring

ABSTRACT

A coil spring includes a helically formed wire having end turn portions at ends thereof, a plurality of first rough surfaces, each including first shot peening indentations formed on a part of a surface of the end turn portions, and a second rough surface including second shot peening indentations formed on the entire surface of the wire except for the first rough surface. The second rough surface has a surface roughness different from that of the first rough surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. application Ser.No. 15/434,519, filed Feb. 16, 2017, which is a Continuation Applicationof PCT Application No. PCT/JP2015/066609, filed Jun. 9, 2015 and basedupon and claiming the benefit of priority from prior Japanese PatentApplication No. 2014-167758, filed Aug. 20, 2014. The entire contents ofall the above-identified applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Present invention described herein relates to a vertical motionimpeller-type shot peening device for performing shot peening to a coilspring and a coil spring subjected to shot peening.

2. Description of the Related Art

Coil springs are used as, for example, suspension springs of asuspension device of a vehicle. To increase durability of such a coilspring, there is a known technique to impart a compressive residualstress to the proximity of the surface of the coil spring by shotpeening. Patent Literature 1 (JP2002-361558A) discloses an example ofconventional shot peening devices. A shot peening device projects shotsto the coil spring from a centrifugal accelerator (impeller) as the coilspring continuously conveyed. Furthermore, as disclosed in PatentLiterature 2 (JP2003-117830A), a shot peening method which producesgreater compressive residual stress by stress peening is also known. Inthe stress peening technique, a coil spring is compressed and shotpeening is performed while a stress is applied thereto.

In the continuous shot peening device disclosed in Patent Literature 1(JP2002-361558A), a coil spring is continuously conveyed in a singledirection and shots are projected from a single direction. In such acontinuous shot peening device, some improvement is required to producea greater compressive residual stress on the entire surface of the coilspring. In the stress peening disclosed in Patent Literature 2(JP2003-117830A), shot peening is performed while the coil spring iscompressed; however, in such a compressed spring coil in the stresspeening process, gaps between adjacent pairs of turn portions of a wirebecome narrower than are in a coil spring in a free state. That is,shots projected from a predetermined direction do not easily enter thegaps between adjacent pairs of turn portions of the wire. Shots hit theouter surface side of the coil spring while the inner surface side ofthe coil spring is partly shadowed by the wire. In the shadows, shots donot hit sufficiently, and the compressive residual stress becomesinsufficient and desired durability cannot be achieved.

BRIEF SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a verticalmotion impeller-type shot peening device which can produce a compressiveresidual stress effective to improve durability of the coil spring and acoil spring subjected to shot peening of the device.

According to one embodiment, a vertical motion impeller-type shotpeening device includes a housing in which a coil spring as a workpieceis accommodated, workpiece holding mechanism, stress applying mechanism,rotation mechanism, projection mechanism, and elevator mechanism. Thehousing includes a first chamber with a workpiece port through which thecoil spring is put in and put out and a second chamber which performsshot peening to the coil spring. The workpiece holding mechanismincludes a lower end turn support contacting a lower end turn portion ofthe coil spring and an upper end turn support contacting an upper endturn portion of the coil spring. The workpiece holding mechanism whichholds the coil spring standing between the lower end turn support andthe upper end turn support.

The stress applying mechanism compresses the coil spring conveyed intothe second chamber between the lower end turn support and the upper endturn support. The rotation mechanism rotates the coil spring in thesecond chamber around its vertical axis. The projection mechanismprojects shots toward the coil spring while the coil spring in thesecond chamber is compressed by the stress applying mechanism androtated by the rotation mechanism. The elevator mechanism moves theprojection mechanism vertically while the shots are projected to thecoil spring.

With the vertical motion impeller-type shot peening device of thepresent invention, a compressive residual stress which is effective forthe improvement of the durability can be produced on the coil spring.The first rough surface including first shot peening indentationsremains in an island shape on the parts of the end turn portion wherethe end turn supports contact thereto. However, the first rough surfaceis formed in the end turn portion which can hold much stress, and thus,there is no disadvantage in the durability even if the first roughsurface is created. Whether or not there has been two-shot peeningincluding first shot peening and second shot peening can be checkedvisually by checking if there is a first rough surface before coatingthe coil spring.

The workpiece holding mechanism may include a turntable and a revolutionmechanism. In the turntable, the lower end turn support is disposed. Therevolution mechanism rotates the turntable around the revolution axissuch that the lower end turn support reciprocates between the firstchamber and the second chamber. Furthermore, the workpiece holdingmechanism may include a pair of the lower end turn supports. The lowerend turn supports may be arranged on the turntable 180° symmetricallywith respect to the revolution axis, and the turntable may be rotated180° at a time by the revolution mechanism.

Furthermore, the projection mechanism may include a first impeller unitand a second impeller unit. The first impeller unit projects shots froma diagonally upward position of the coil spring in the second chamberand moves vertically with respect to the coil spring. The secondimpeller unit projects shots from a diagonally downward position of thecoil spring and moves vertically with respect to the coil spring.

Furthermore, the housing may include a first hinge mechanism and asecond hinge mechanism. The first hinge mechanism rotatably supports thefirst impeller unit in either an open position or a close position withrespect to the second chamber. The second hinge mechanism rotatablysupports the second impeller unit in either an open position or a closeposition with respect to the second chamber.

According to an embodiment, a coil spring comprises a helically formedwire and has end turn portions at its ends. The coil spring includes afirst rough surface formed on a part of the surface of the end turnportions, and a second rough surface formed on the entire surface of thewire except for the first rough surface. The first rough surfaceincludes first shot peening indentations. The second rough surfaceincludes second shot peening indentations of which surface roughness isdifferent from that of the first rough surface. For example, the firstrough surface is scattered in a plurality of locations in an islandshape on the end turn portion at intervals in the winding direction.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a perspective view of a coil spring of an embodiment.

FIG. 2 shows a part of the coil spring of FIG. 1 in an enlarged manner.

FIG. 3 is a perspective view of the coil spring before coating.

FIG. 4 shows a part of the before-coating coil spring of FIG. 3 in anenlarged manner.

FIG. 5 shows an example of each process in a manufacturing process ofthe coil spring of FIG. 1.

FIG. 6 is a perspective view schematically showing a continuous shotpeening device.

FIG. 7 is a front view showing a part of a vertical motion impeller-typeshot peening device of an embodiment.

FIG. 8 is a vertical cross-sectional view of the vertical motionimpeller-type shot peening device of FIG. 7.

FIG. 9 is a horizontal cross-sectional view of the vertical motionimpeller-type shot peening device of FIG. 7.

FIG. 10 is a horizontal cross-sectional view of the vertical motionimpeller-type shot peening device of FIG. 7 in which an impeller unit ismoved in an open position.

DETAILED DESCRIPTION OF THE INVENTION

A coil spring and a vertical motion impeller-type shot peening device ofan embodiment will be explained hereinafter with reference to FIGS. 1 to10.

FIG. 1 shows an example of a coil spring 10 which is coated. FIG. 2shows a part of the coated coil spring 10 (S1 in FIG. 1) in an enlargedmanner. The coil spring 10 includes element wire (wire) 11 formedhelically. The surface of the wire 11 is covered with a rust-proofcoating (paint coat) 12. End turn portions 10 a and 10 b are formed atthe ends of the coil spring 10. A helically-shaped effective portion 10c is formed between the end turn portions 10 a and 10 b to deformcorresponding to a compressive load applied thereto.

In FIG. 1, the lower end turn portion 10 a is less than one turn (forexample, 0.6 turn) from the lower end of the wire 11. In, for example, avehicle suspension device, the lower end turn portion 10 a contacts alower spring seat. In FIG. 1, the upper end turn portion 10 b is lessthan one turn (for example, 0.6 turn) from the upper end of the wire 11.In, for example, a vehicle suspension device, the upper end turn portion10 b contacts an upper spring seat. A torsional stress is produced onthe wire 11 when the effective portion 10 c is deformed by a compressiveload applied on the coil spring 10.

In the example, the coil spring 10 is formed as a cylindrical coilspring. However, the coil spring may be changed in various types such asbarrel-type coil spring, hourglass-type coil spring, tapered coilspring, irregularly-pitched coil spring to conform to types of thesuspension device. The wire 11 is formed of a spring steel of whichcross-section is a circle. In a coil spring for a suspension of anautomobile, the wire 11 generally has a diameter of 8 to 21 mm; however,the diameter thereof is, of course, optional.

The spring steel used for the wire 11 is not limited to a specific kind,and there is SAE9254 which complies US “Society of AutomotiveEngineers”, for example. Chemical compositions (mass %) of SAE9254 areC: 0.51 to 0.59, Si: 1.20 to 1.60, Mn: 0.60 to 0.80, Cr: 0.60 to 0.80,S: 0.040 at maximum, P: 0.030 at maximum, Fe: remaining. The steel maybe a super high strength spring steel.

FIG. 3 shows the coil spring 10 before coating (formation of a coatingfilm). FIG. 4 shows a part of the coil spring 10 before coating (S2 inFIG. 3) in an enlarged manner. As in FIGS. 3 and 4, on a part of the endturn portions 10 a and 10 b before coating, a first rough surface 21 isformed (schematically shown in thicker dots in FIGS. 3 and 4). The firstrough surface 21 includes first shot peening indentations 20. The firstshot peening indentations 20 include a large number of microscopicasperities.

There are several (three or four) first rough surfaces 21 on the lowerend turn portion 10 a and the upper end turn portion 10 b. The firstrough surfaces 21 are scattered in an island shape at intervals on theend turn portions 10 a and 10 b in the winding direction thereof. Thefirst shot peening indentations 20 are formed on the entire surface ofthe wire 11 in a first shot peening process explained hereinafter. Inthe first shot peening process, first shots SH1 are projected to a coilspring 12 by, for example, a continuous shot peening device 40 (shown inFIG. 6). A second rough surface 31 (schematically shown in FIGS. 3 and 4in thinner dots) is formed on the surface of the wire 11 except for thepart where the first rough surface 21 is formed. The second roughsurface 31 includes second shot peening indentations 30. The second shotpeening indentations 30 include a large number of microscopicasperities.

The second shot peening indentations 30 are formed on the entire surfaceof the wire 11 except for the first rough surface 21 by a verticalmotion impeller-type shot peening device 50. The vertical motionimpeller-type shot peening device 50 will be explained later. Thesurface roughness of the second rough surface 31 is different from thatof the first rough surface 21. The surface roughness differs dependingon the conditions of shot peening. The roughness of the second roughsurface 31 is less than that of the first rough surface 21. For example,the first rough surface 21 has its maximum height of 30 to 50 μm whilethe second rough surface 31 has its maximum height of 20 to 30 μm.

FIG. 5 shows an example of a manufacturing process of the coil spring10. In a forming process S1 of FIG. 5, the wire 11 is formed helicallyby a coiling machine. In a heat treatment process S2, tempering andannealing of the wire 11 are performed to remove a distortion stressproduced in the wire 11 by the forming process S1. For example, in theheat treatment process S2, the wire 11 is heated to 400 to 450° C. andthen is cooled slowly. In a hot setting process S3, hot setting isperformed in a warm (250 to 350° C.) using a residual heat of the heattreatment process S2. In the hot setting, a load in the axial directionis applied to the coil spring 10 in the warm for a certain period by apressure applying device.

Then, in a first shot peening process S4, first shot peening isperformed in the warm. In the first shot peening process S4, first shots(for example, a large size cut wire of which grain diameter is 1.1 mm)are used. Note that the shot size may vary (for example, between 0.87and 1.2 mm). The first shots are projected to the entire surface of thecoil spring 10 in a process temperature of 250 to 300° C. by thecontinuous shot peening device 40 which is schematically shown in FIG.6. The speed of projection of the first shots are, for example, 77 m/s.

FIG. 6 shows an example of the continuous shot peening device 40. Theshot peening device 40 includes a pair of rotatable rollers 41 and 42and a centrifugal accelerator (impeller) 43. The coil spring 10 disposedon the rollers 41 and 42 is rotated by the rollers 41 and 42 andcontinuously moves in the direction of arrow F in the figure. Thecentrifugal accelerator (impeller) 43 projects the first shots SH1 tothe coil spring 10.

With the first shot peening, a compressive residual stress is producedto a relatively deep position from the surface of the coil spring 10.Furthermore, an oxide film (mill scale formed in the heat treatment) onthe surface of the wire 11 is removed by the first shot peening.Furthermore, through the first shot peening, first shot peeningindentations 20 (part of which is schematically shown in FIG. 4) areformed on the surface of the wire 11. Therefore, in a coating processS7, paint can be easily adhered to the surface of the wire 11 of thecoil spring 10 which has been subjected to the first shot peening.

In a second shot peening process S5 of FIG. 5, second shot peening (warmstress shot peening) is performed in a temperature lower than that ofthe first shot peening process S4 (for example, 200 to 250° C.). Thesecond shot peening is performed while the coil spring 10 is compressedby the vertical motion impeller-type shot peening device 50 shown inFIGS. 7 to 10. In the second shot peening process S5, second shots SH2(for example, a small size cut wire of which grain diameter is 0.4 to0.7 mm) is projected to the entire surface of the coil spring 10. Thesize of second shots SH2 are less than that of the first shots SH1 usedin the first shot peening process S4.

Through the second shot peening process S5 (warm stress shot peening),the absolute value of the compressive residual stress in the proximityof the surface of wire 11 can be increased. Furthermore, the small sizesecond shots SH2 are projected while the coil spring 10 is heated to thewarm temperature range and is compressed. Therefore, the compressiveresidual stress in the proximity of the surface of coil spring 10 can beincreased effectively and the surface roughness of wire 11 can beimproved (the surface roughness can be decreased). Therefore, thedurability of the coil spring 10 can further be increased.

After the second shot peening process S5, a setting process S6 isperformed if necessary. Through the setting process S6, the length ofthe coil spring when no load is applied thereto (free length) isadjusted. In the setting process S6, the creep resistance of coil spring(sag resistance) can be improved. Note that the setting process S6 maybe omitted. Then, in a coating process S7, a rust-proof paint is appliedto the entirety of the coil spring by electrostatic coating or the like.Then, lastly, a quality inspection is performed and the coil spring 10is completed.

FIGS. 7 to 10 show the vertical motion impeller-type shot peening device50. The vertical motion impeller-type shot peening device 50 is used inthe second shot peening process S5. FIG. 7 is a front view showing apart of the vertical motion impeller-type shot peening device 50. FIG. 8is a vertical cross-sectional view, and FIGS. 9 and 10 are horizontalcross-sectional views.

The vertical motion impeller-type shot peening device 50 includes ahousing 51 in which the coil spring (workpiece) 10 is accommodated,workpiece holding mechanism 52, projection mechanism 57, first elevatormechanism 58, and second elevator mechanism 59. The workpiece holdingmechanism 52 holds the coil spring 10 as standing substantiallyvertically. The projection mechanism 57 includes a first impeller unit55 and a second impeller unit 56 which project shots SH2 to the coilspring 10. The first elevator mechanism 58 moves the first impeller unit55 vertically. The second elevator mechanism 59 moves the secondimpeller unit 56 vertically.

For example, the first elevator mechanism 58 and the second elevatormechanism 59 include servo motors 58 a and 59 a (shown in FIG. 8) ofwhich rotation is controlled by a controller and ball screws 58 b and 59b rotated by the servo motors 58 a and 59 a. Based on the direction andamount of rotation of the servo motors 58 a and 59 a, the impeller units55 and 56 independently move vertically at constant strokes Y1 and Y2,respectively.

As shown in FIGS. 8 and 9, a first chamber 61, second chamber 62, andmiddle chambers 63 and 64 which are disposed between the chambers 61 and62 are formed inside the housing 51. A workpiece port 65 is formed inthe first chamber 61. The workpiece port 65 is an aperture through whichthe coil spring 10 is put in and out the first chamber 61 from theoutside the housing 51.

The second chamber 62 includes a projection port 55 a of the firstimpeller unit 55 and a projection port 56 a of the second impeller unit56. With the first impeller unit 55 and the second impeller unit 56, theprojection mechanism 57 is structured. Shot SH2 are projected to thecoil spring 10 from the projection ports 55 a and 56 a. That is, thesecond shot peening is performed by projecting shots SH2 to the coilspring 10 in the second chamber 62.

As shown in FIGS. 9 and 10, partition walls 70 and 71 are providedbetween the first chamber 61 and the middle chambers 63 and 64.Partition walls 72 and 73 are provided between the second chamber 62 andthe middle chambers 63 and 64. Seal walls 74 and 75 are formed in themiddle chambers 63 and 64. The seal walls 74 and 75 keep the shots SH2projected in the second chamber 62 from going to the first chamber 61.

As shown in FIG. 7, the workpiece holding mechanism 52 includes aturntable 79, motor-equipped revolution mechanism 80, and a pair ofworkpiece holders 81 and 82. The turntable 79 rotates around arevolution axis X1 extending in the vertical direction. The revolutionmechanism 80 intermittently rotates the turntable 79, 180° at a timearound the revolution axis X1 in either first direction R1 or seconddirection R2 (shown in FIG. 9). The workpiece holders 81 and 82 aredisposed on the turntable 79.

The workpiece holders 81 and 82 each include lower end turn supports 84and 85 contacting the lower end turn portion 10 a of the coil spring 10.The workpiece holders 81 and 82 are positioned 180° symmetrically aboutthe revolution axis X1. In the rear side of the workpiece holders 81 and82, a pair of backup plates 86 and 87 are disposed. The backup plates 86and 87 receive shots projected to the coil spring 10 in the secondchamber 62.

For example, the lower end turn support 84 provided with the workpieceholder 81 includes a plurality of pawls (for example, four pawls). Thepawls are disposed on the workpiece holder 81 in its peripheraldirection at regular intervals. The pawls are formed in either a U-shapeor an L-shape to support the end turn portion 10 a inserted from theabove. The lower end turn support 85 provided with the other workpieceholder 82 includes a plurality of pawls (for example, four pawls). Thepawls are disposed on the workpiece holder 82 in its peripheraldirection at regular intervals. As with the pawls of the workpieceholder 81, the pawls are formed in either a U-shape or an L-shape tosupport the end turn portion 10 a inserted from the above.

The turntable 79 is rotated by the revolution mechanism 80 (shown inFIG. 7). The revolution mechanism 80, while placing the workpiece holder81 in the center of the first chamber 61 (workpiece in-out positionshown in FIG. 9), moves the other workpiece holder 82 in the center ofthe second chamber 62 (shot peening position). Furthermore, therevolution mechanism 80, while moving the workpiece holder 81 in thecenter of the second chamber 62 (shot peening position), places theother workpiece holder 82 in the center of the first chamber 61(workpiece in-out position). That is, the revolution mechanism 80intermittently rotates the turntable 79, 180° at a time around therevolution axis X1 in either first direction R1 or second direction R2(shown in FIG. 9).

In the present embodiment, the workpiece holders 81 and 82 of theworkpiece holding mechanism 52 are positioned on the turntable 79, 180°symmetrically. The pair of workpiece holders 81 and 82 include the lowerend turn supports 84 and 85, respectively. The turntable 79 isintermittently rotated 180° at a time around the revolution axis X1 bythe revolution mechanism 80. That is, the revolution mechanism 80rotates the turntable 70 about the revolution axis X1 such that thelower end turn supports 84 and 85 reciprocate over the first chamber 61and the second chamber 62.

Furthermore, the vertical motion impeller-type shot peening device 50 ofthe present embodiment includes a stress applying mechanism 90 and arotation mechanism 100. The stress applying mechanism 90 compresses thecoil spring 10 held by the workpiece holding mechanism 52. The rotationmechanism 100 allows the coil spring 10 held by the workpiece holdingmechanism 52 to rotate around the vertical axes (rotation axes) X2 andX3.

The stress applying mechanism 90 includes presser members 91 and 92 andpresser drivers 93 and 94 which are formed of, for example, a fluidcylinder. The presser members 91 and 92 are arranged just above theworkpiece holders 81 and 82. The presser drivers 93 and 94 move thepresser members 91 and 92 vertically, respectively. Upper end turnsupports 95 and 96 are provided with the presser members 91 and 92. Theupper end turn supports 95 and 96 contact the upper end turn portion 10b of the coil spring 10.

The upper end turn support 95 is provided with the presser member 91,and for example, the upper end turn support 95 includes a plurality ofU-shaped or L-shaped pawls (for example, four pawls) which can supportthe end turn portion 10 b. The pawls are disposed on the presser member91 in its peripheral direction at regular intervals. The upper end turnsupport 96 is provided with the other presser member 92, and forexample, the upper end turn support 96 includes a plurality of U-shapedor L-shaped pawls (for example, four pawls) which can support the endturn portion 10 b. The pawls are disposed on the presser member 92 inits peripheral direction at regular intervals.

The presser drivers 93 and 94 move the presser members 91 and 92vertically. The presser drivers 93 and 94 compress the coil spring 10while placing the presser members 91 and 92 at the lower stroke end.Furthermore, the presser drivers 93 and 94 release the pressure to thecoil spring 10 while placing the presser members 91 and 92 at the upperstroke end. That is, the presser drivers 93 and 94 are configured tomove vertically in a stroke to both compress the coil spring 10 and torelease the pressure to the coil spring 10.

The rotation mechanism 100 which rotates the coil spring 10 includes alower rotator 101 and an upper rotator 102. The lower rotator 101rotates the workpiece holders 81 and 82 around the vertical axes X2 andX3. The upper rotator 102 rotate the presser members 91 and 92 aroundthe vertical axes X2 and X3. The lower rotator 101 and the upper rotator102 rotate in synchronization in the same direction at the samerevolution by the drivers and timing belts. The drivers are, forexample, servo motors controlled by a control circuit.

The projection mechanism 57 projects shots SH2 to the coil spring 10.The projection mechanism 57 includes the first impeller unit 55 and thesecond impeller unit 56 which can move vertically. As shown in FIG. 8,the first impeller unit 55 projects shots SH2 to the coil spring 10 inthe second chamber 62 from a diagonally upward position. Furthermore,the first impeller unit 55 moves vertically by the first elevatormechanism 58. The second impeller unit 56 projects shots SH2 to the coilspring 10 in the second chamber 62 from a diagonally downward position.Furthermore, the second impeller unit 56 moves vertically by the secondelevator mechanism 59.

FIGS. 9 and 10 are horizontal cross-sectional views in which the firstimpeller unit 55 and the second impeller unit 56 viewed from the above.The first impeller unit 55 includes an impeller (wing wheel) 111 rotatedby a motor 110 and a distributor 112 which supplies shots SH2 in theimpeller 111. The second impeller unit 56 includes an impeller 116rotated by a motor 115 and a distributor 117 which supplies shots SH2 inan impeller 116.

As shown in FIG. 9, as being viewed from the above, the coil spring 10is disposed in the second chamber 62. Shots are projected to the coilspring 10 from each of the first impeller unit 55 and the secondimpeller unit 56. Lines P1 and P2 in FIG. 9 indicate shot projectiondirections toward the center of the coil spring 10. The first impellerunit 55 and the second impeller unit 56 are arranged such that lines P1and P2 form an angle θ which is less than 180° (for example, 60°).Therefore, the first impeller unit 55 and the second impeller unit 56can project shots SH2 to the coil spring 10 without interfering eachother.

The first impeller unit 55 is supported such that it can move verticallyalong a vertical motion guide member 130 provided with the side ofhousing 51. The first impeller unit 55 reciprocates by the firstelevator mechanism 58 from a neutral position N1 shown in FIG. 8 to goover an ascend position A1 and a descend position B1. The first elevatormechanism 58 includes, for example, drivers such as servo motor 58 a andball screw 58 b.

Furthermore, the first impeller unit 55 can open/close about the firsthinge mechanism 131 provided with the housing 51. That is, the firstimpeller unit 55 can rotate over a close position shown in FIG. 9 and anopen position shown in FIG. 10. Then, the first impeller unit 55 issecurely locked while the second chamber 62 is closed (the closeposition in FIG. 9). Furthermore, the first impeller unit 55 can be setin a state where the second chamber 62 is released (the open position inFIG. 10) for maintenance purpose or the like.

The second impeller unit 56 is supported such that it can movevertically by a vertical-motion guide member 140 provided with the sideof housing 51. The second impeller unit 56 reciprocates by the firstelevator mechanism 59 from a neutral position N2 shown in FIG. 8 to goover an ascend position A2 and a descend position B2. The secondelevator mechanism 59 includes, for example, drivers such as servo motor59 a and ball screw 59 b.

Furthermore, the second impeller unit 56 can open/close about the secondhinge mechanism 141 provided with the housing 51. That is, the secondimpeller unit 56 can rotate over a close position shown in FIG. 9 and anopen position shown in FIG. 10. Then, the second impeller unit 56 issecurely locked while the second chamber 62 is closed (the closeposition in FIG. 9). Furthermore, the second impeller unit 56 can be setin a state where the second chamber 62 is released (the open position inFIG. 10) for maintenance purpose or the like.

As shown in FIG. 10, the first impeller unit 55 and the second impellerunit 56 are moved to the open positions around the hinge mechanism 131and 141, respectively. In this state, the second chamber 62 is releasedand the inside of second chamber 62 can be seen from the outside ofhousing 51. At the same time, the inside of first impeller unit 55 andthe inside of second impeller unit 56 can be seen through the projectionports 55 a and 56 a. Therefore, the maintenance of the second chamber 62and the impeller units 55 and 56 can be performed.

Now, the second shot peening process S5 (as shown in FIG. 5) performedwith the vertical motion impeller-type shot peening device 50 of thepresent embodiment will be explained.

Initially, a first coil spring 10 is disposed on the workpiece holder 81in the first chamber 61. The coil spring 10 drawn in the left of FIG. 7is in a state where a compression load is not applied (free state) andthe length of coil spring 10 (free length) is L1. On the entire surfaceof coil spring 10, first shot peening indentations 20 are formedpreliminarily through the first shot peening process S4 (shown in FIG.5).

The workpiece holder 81 is stopped in the workpiece in-out position inthe first chamber 61. The coil spring 10 is disposed on an end turnsupport 84 of the workpiece holder 81. Then, the presser member 91 islowered to the lower stroke end. In that state, the coil spring 10 iscompressed to a length L2 between the lower end turn support 84 and theupper end turn support 95. Thereby, a torsional stress is applied to thecoil spring 10. Then, the turn table 79 rotates 180°, the coil spring 10is conveyed in the shot peening position of the second chamber 62together with the workpiece holder 81. At the same time, the otherworkpiece holder 82 is moved to the first chamber 61. Therefore, in thefirst chamber 61, a second coil spring 10 can be disposed on theworkpiece holder 82.

In the second chamber 62, the compressed coil spring 10 is rotated bythe rotation mechanism 100. The second shot peening is performed to therotating coil spring 10 by the first impeller unit 55 and the secondimpeller unit 56 which move vertically. In the second shot peening, thefirst impeller unit 55 and the second impeller unit 56 move verticallyin synchronization. Since the compressed coil spring 10 rotates, thesecond shots SH2 are projected to the entire surface of wire 11including the end turn portions 10 a and 10 b and the effective portion10 c. By performing the second shot peening with the stress applied, thecompressive residual stress can increase in the proximity of the surfaceof coil spring 12.

In the present embodiment, the second shot peening is performed whilethe coil spring 10 is compressed. That is, the shots SH2 are projectedwhile gaps between adjacent pairs of turn portions of the wire becomeless than that in the free state. Therefore, if the shots SH2 areprojected from a single direction, the shots may miss a part of the coilspring 10. In consideration of this point, a pair of the impeller units55 and 56 each move vertically, the coil spring 10 rotates, and theshots SH2 are projected to the coil spring 10 from both diagonallyupward and downward positions. Therefore, shots can sufficiently hit theentirety of the coil spring 10.

In the second shot peening, the second shots SH2 are projected over thefirst shot peening indentations by the vertical motion impeller-typeshot peening device 50. Therefore, the second shots SH2 hit the surfaceof wire 11 except for the positions contacting the end turn supports 84,85, 95, and 96. Thereby, the second rough surface 31 is formed on thesurface of wire 11 except for the positions contacting the end turnsupports 84, 85, 95, and 96. The second rough surface 31 includes alarge number of second shot peening indentations 30 of which surfaceroughness is less than that of the first shot peening indentations 20(shown in FIGS. 3 and 4). The second shots SH2 do not hit the positionscontacting the end turn supports 84, 85, 95, and 96. Therefore, thefirst rough surface 21 including the first shot peening indentations 20is scattered on a part of the end turn portions 10 a and 10 b.

After the second shot peening is performed in the second chamber 62, theturntable 79 rotates 180°. Thereby, the coil spring 10 on the workpieceholder 81 returns to the first chamber 61 from the second chamber 62. Atthe same time, a second coil spring 10 held by the other workpieceholder 82 is conveyed in the second chamber 62.

When the coil spring 10 on the workpiece holder 81 returns to the firstchamber 61 from the second chamber 62, the presser member 91 moves up.The coil spring 10 is extracted to the outside of first chamber 61 fromthe workpiece port 65. The second coil spring 10 is conveyed from thefirst chamber 61 to the second chamber 62. The second coil spring 10 is,as with the first coil spring 10, subjected to the second shot peeningby the first impeller unit 55 and the second impeller unit 56 in thesecond chamber 62.

Here, hypothetically, there is some failure in the manufacturing processof the coil spring of FIG. 5, and the first shot peening process S4 orthe second shot peening process S5 is not performed. In that case, theentire surface of coil spring 10 only includes the shot peeningindentations of the same surface roughness and first rough surfaces 21scattered in an island shape are not formed. Therefore, as to the coilspring 10 before the coating, whether or not both the first shot peeningand the second shot peening are performed can be checked by seeing thefirst rough surfaces 21 scattered in an island shape are formed on theend turn portions 10 a and 10 b. That is, whether or not two kinds ofshot peenings have been performed can be checked. After the coating ofthe coil spring, whether or not there is a first rough surface 21 ischecked by peeling off either one of the end turn portions 10 a and 10b.

The first rough surface 21 only includes the first shot peeningindentations 20. Therefore, the increase of residual stress obtained bythe second shot peening indentations 30 cannot be expected in the firstrough surface 21. However, the first rough surfaces 21 are scattered inan island shape on the end turn portions 10 a and 10 b which can holdmuch stress. Therefore, even if there are first rough surfaces 21remaining on a part of the coil spring 10, the durability of coil spring10 is not harmed thereby.

The vertical motion impeller-type shot peening device 50 of the presentembodiment includes the first chamber 61 and the second chamber 62. Theturntable 79 intermittently rotates 180° at a time and a pair ofworkpiece holders 81 and 82 are alternatively conveyed in the firstchamber 61 and the second chamber 62. Therefore, while an operatorhandles the coil spring 10 in the first chamber 61, shot peening isperformed in the second chamber 62. Therefore, the second shot peeningprocess S5 can be performed efficiently to a plurality of coil springs10.

The vertical motion impeller-type shot peening device of the presentapplication can be applied to a coil spring other than suspension coilspring. Furthermore, to achieve the present invention, various models,structures, and arrangements of elements used in the vertical motionimpeller-type shot peening device such as specific shapes and structuresof the housing, workpiece holding mechanism, stress applying mechanism,rotation mechanism, projection mechanism, and elevator mechanism can bechanged arbitrarily. For example, there may be only one workpiece holderincluding lower end turn supports or may be three or more workpieceholders. Furthermore, the coil spring of the present embodiment can beused various purposes other than the vehicle suspension device.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims.

What is claimed is:
 1. A coil spring comprising a helically formed wireand having end turn portions at ends thereof, the coil springcomprising: a plurality of first rough surfaces, each including firstshot peening indentations formed on a part of a surface of the end turnportions; and a second rough surface including second shot peeningindentations formed on the entire surface of the wire except for thefirst rough surface, the second rough surface having a surface roughnessdifferent from that of the first rough surface.
 2. The coil spring ofclaim 1, wherein the surface roughness of the second rough surface isless than that of the first rough surface.
 3. The coil spring of claim1, wherein the first rough surfaces are scattered at intervals on theend turn portions in a winding direction.
 4. The coil spring of claim 3,wherein the first rough surfaces on a lower end turn portion are formedon a lower surface of the lower end turn portion in a state where thecoil spring is standing substantially vertically.
 5. The coil spring ofclaim 4, wherein the first rough surfaces on an upper end turn portionare formed on an upper surface of the upper end turn portion in a statewhere the coil spring is standing substantially vertically.